Volumes 3 and 4 are missing, but everything else is there, albeit with a terrible numbering system. It'd be better just to label the chapters sequentially from 1 to whatever the last one is rather than getting chapter gap warnings when moving to a new volume.
Too bad the anime didn't get more seasons, it was really good and really underrated. I think them overemphasizing the kissing the pillow scene in the first episode turned too many people off. If they'd just kept it more low key, like the manga, more people would've probably kept on watching instead of dropping it right there.
I almost dropped it, but I'm glad I went back, aside from that bit (which was less cringe the second time I tried to watch it) it was a stellar anime and deserved more than one season.